I've lurked here for a long time and I often find that many shared experiences leave me with more questions than answers.
Im sorry for what you're going through, it doesn't sound like much fun and believe me I am hardly an apologist for DH.
From what I've been able to gather from your posts he is someone you love and have been close friends with for 8 years --- been together with eachother nearly 36 months, currently pregnant with your second child and your first child is 20 months old ..... So you both had to deal with the idea of being parents at most 6 months into your relationship
You both have had poly relationships in the past but this one was exclusive from the outset.
You have difficulty with him being with other women, he does not have difficulty with you being with other women ( not sure about men) ... And before all these revelations you benifited from the relationship with your friend and if I understood correctly some of this benefit spilled over into your relationship with DH In a positive way.
You brushed over a couple things including a miscarriage, SS , depression etc
You also said, again if I am interpreting this correctly that you were fine with him having sex with this friend, as long as it didn't become romantic and in some part because well he didn't have any trouble with you sleeping with her, she was leaving soon etc.
Now as far as the lieing, cheating etc from both of them ... His and perhaps your immaturity -- can it be worked out I dunno and I expect neither does anyone else.
I believe this can be a wonderful place for ideas and support, I hope you find that and I sincerely wish you the best in all of this.... I mean as much as one can when so far removed with anonymity.
I do think in addition to his cheating and dishonesty you have a number of things to work out that would be true even if none of that had happened. That doesn't mean I'm dismissing it, and by the numbers can you trust him again --- I don't think I could --- but the larger question might be can you trust yourself
It may be helpful to learn how you (and he decided) that this relationship was going to be exclusive from the beginning...... And does that mean you had a discussion about it before you entered into a romantic relationship, or before you decided t get married? I mean given your collective backgrounds and experience in what you term poly relationships .... And then you decided that an exclusive mono relationship would be best .... Why is that?
Given what you pursued with your girlfriend inside of the past 36 months does it really seem like mono is the best choice for you .... Or is it Mostly the best choice for him from your point of view? I guess what I'm getting at is would you really like to have other romantic meaningful relationships in addition to the one with your husband .... But can't tolerate him having the same experiences? And if so why do you think that is?
It also sounds like you have been pregnant 3 times in the past 30 months or so (counting the miscarriage) .... And have been married for at most 36 months. That can be a beautiful thing but as you know it comes with some hardship as well.
It's really easy for me to say, you shouldn't walk away from DH ... You should run. It's also easy to focus on his colossal errors, dishonesty, and immaturity. At the end of the day, you're the one that loves him so I can only assume he has some redeeming qualities you have failed to mention.
Whatever you decide however I would think it unwise to go to all the trouble, effort and heartbreak of addressing this ( clean break, reconciliation or whatever form that takes) ... And not pay more attention to what it is that you honestly need from relationships and why.
I wish you well.